I am an adult male who doesn't know much about astronomy, but would like to learn more. Recently I heard that scientists have discovered evidence that expansion of the universe is speeding up and that this is a very surprising finding. But is it possible that this is not surprising at all. I read about a year ago of an old theory that the red shift that is used to justify the idea that the galaxies are moving away from each other has another possible explanation that is even older, known as the tired light theory. Doesn't this new finding support this old theory, and if not why not?

There are many problems with the tired light theory, the main one being that there is no known way for photon's to lose energy (without also changing in other observable ways) that is not equivalent to the universe expanding and the photons expanding with it. The main argument against conventional cosmology (in which the universe is homogeneously expanding) and for tired light theory seems to be that if the universe were expanding then we (and all of our measuring sticks) would be expanding along with it so we wouldn't be able to measure it. This is a common misconception. Locally, objects can stop expanding with the universe if the forces holding them together are strong enough. Those forces might be the forces holding the atoms in your body together or the force of gravity holding the stars together in a galaxy. Take a look at the below archived questions on this topic.

The observed accleration of the expansion of the universe was a big surprise to conventional cosmology and we are still working on theories which can include it! They are a little strange sounding, requiring a 'cosmological constant' or 'dark energy', which we don't understand, but the surprising thing is that given this, they predict the growth of structure in our universe and the abundances of elements, and the temperature of the microwave background radiation (among other things) extremely well. With the amount of observational evidence piling up behind them, these theories are looking in good shape.

What an acceleration of the expansion says in tired light theory I have not given much thought to until now. The observation that lead to the conclusion that the expansion was accelerating was that Supernovae appeared to be further away (and therefore dimmer) than expected if the universe was expanding with a constant rate. What I mean by this is that they have a lower redshift than predicted given their distance measured in a different way - so are further away than the redshift suggests. I think that in tired light theory this would have to be interpreted as being due to a non-constant loss of energy by the photons on their way to us. Since there is no know mechanism for photons to lose energy without interacting with other particles (and therefore changing in other observable ways) in a non-expanding universe even in a constant way, how you would find a variable mechanism is anyone's guess!

You might guess that I don't think much of tired light theory. Check out this somewhat technical website about errors in the theory. There's another one here which has some mention of tired light theory.

My main problem with it is that the arguments that say it is needed and the expansion of the universe is the wrong conclusion to draw do not consider all of the available evidence. We don't think that the universe is expanding and started in a 'big bang' just because we observe ALL the galaxies to be moving away from us, there is other very convincing evidence that supports it too. The currently favoured cosmological model explains the formation of structures we observe in the universe, both now and in the distant past. It also explains the temperature of the cosmic microwave background and the amounts of different elements that were formed in the early universe. My only conclusion is that the supporters of tired light theory do not understand that evidence, which is understandable as it has taken real astronomers years of study to become astronomers.

Thank you for your speedy reply. Although I can't say I completely understood it. It is clear that the tired light theory is not based on good science. That leads me to believe that the publication I read in which the tired light theory was defended on two separate occasions over a six month period must have a hidden agenda, much like some religious conservatives that come up with theories to discredit evolution.

About the Author

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Curious Minds Online

We have 4748 guests and no members online

How Many Were Here?

Total page views since 1997

96770535

Who are We?

Ask an Astronomer is run by volunteers in the Astronomy Department at Cornell University. Most of us are graduate students at Cornell, and all of us do this voluntarily, in our own time, fitting it in around our other work. Please take the time to browse our site and first try to use the resources online to find an answer to your question.